The Fleet "Computer Correction Factors" or CCF's were developed for each type of ammunition and then updated as the ammunition was updated. These were determined by firing rounds from cannons that had various numbers of rounds fired from them (New, 50, 100, 200) Those averages were applied across the fleet.
Prior to firing any of the "Main Gun" Gunnery Tables (in my day TTVIA/B, TTVIIA/B, TTIIIA/B (Crew Qualification), TTIXA/B, TTXA/B (Section Qualification), TTXIA/B, TTXIIA/B (Plt Qualification),A/B being Day/Night) US Crews fired "Calibration or Live Fire Accuracy Screening Test" (LFAST) to ensure that the Fire Control System was functioning as designed, the Crews had Boresighted the weapon systems correctly, and was used to zero the tank mounted Machineguns. FCS/Boresight Data was recorded and compared to the tanks previous data prior to any rounds being fired. If the data was within tolerence (.3mil) of the previous sampling, the crew was allowed to fire. If not, the Master Gunner would have the crew redo the tanks Boresight, and submit another data sheet. If the data fell within the .3mil tolerence, the crew fired their Calibration rounds. If not, the Master Gunner had to determine the cause of the deviance.
The Main Gun Target was at 1500m and one round Sabot and one round HEAT were fired, the hit plotted on a data sheet. The round was fired using what on the M1 series is called "Normal Mode" (ie using all the FCS capabilities).
The reason US Armor doctrine changed from "Zeroing" each tank individually to a "Fleet" Zero was twofold.
1) The Army was spending hugh amounts of $$$ on training ammunition "chasing bullets".
2) (and you'll appreciate this) A British research paper on tank gun accuracy produced in the late 50's established that there were some 11 "Variable Bias's" (Vehicle Cant, Optical Path Bending, Windage Jump, Fire Control Nominality, Air Temp, Range to Target, Muzzle Velocity Variation, Gun Jump, Ammunition Propellant Temp, Crosswind, Zeroing Procedures) that come into play with each round fired that a tank crew can not control, that effect the strike of any given round.
Also, all US tanks have a Gunners Auxillary Sight (GAS) with graduated reticles for the different rounds if the primary FCS is disabled.